My lights have been dim inside my car lately and i'm thinking it's the alternator but i'm not sure.

My car starts fine and i just bought a battery like a month or two ago and it's an optima RED top.

anyway my lights are dim inside and they seem to brighten up when the car is moving but don't return to there dim state when it's not, it just stays dim upon start-up and get's brighter when i drive and stays alright, kinda worried what could it be???

Trance.